Biology MCQs with Answers and Explanations

A female exhibits an X-linked recessive trait. What must be true of her parents’ genotypes?

A. Her father must have the trait, and her mother must be at least a carrier
B. Both parents must be affected
C. Only her mother is affected
D. Either parent could have the trait independently

Explanation:
For a daughter to be homozygous for an X-linked recessive allele, her father must have the allele (so he is affected) and her mother must carry at least one mutant X.

A mother carries a mutated X-linked recessive gene but is unaffected. What is the probability that a son of hers will express the trait?

A. 0%
B. 25%
C. 50%
D. 100%

Explanation:
A carrier mother (X^mX) has a 50% chance of passing the mutant X to a son. If he receives it, he will express the recessive trait (since sons inherit only one X chromosome).

A man has an X-linked recessive disease and marries an unaffected woman who is not a carrier. What is the outcome for their children?

A. All daughters will be carriers; none of the sons will be affected
B. All daughters will be affected, sons unaffected
C. All sons will be affected; no daughters will be carriers
D. None of their children will inherit the disease allele

Explanation:
The father passes his mutant X to all daughters, making them carriers (but not affected). He passes his Y to his sons, so no son is affected or a carrier.

What is the genotype of a person with blood type O in the ABO system?

A. IᴬIᴬ or Iᴬi
B. IᴮIᴮ or Iᴮi
C. ii
D. IᴬIᴮ

Explanation:
Type O blood has no A or B antigen and is genotypically ii (both alleles are the recessive i).

In the ABO blood group, what blood type is expressed by an individual with genotype IᴬIᴮ?

A. Type A
B. Type B
C. Type AB
D. Type O

Explanation:
Alleles Iᴬ and Iᴮ are codominant, so the IᴬIᴮ genotype produces both A and B antigens on red cells, yielding blood type AB.

In a large, randomly mating population under Hardy–Weinberg equilibrium, 9% of individuals display a recessive trait. What percentage are heterozygous carriers?

A. 18%
B. 42%
C. 9%
D. 81%

Explanation:
If q²=0.09, then q=0.30 and p=0.70. Heterozygous frequency = 2pq = 2(0.70)(0.30) = 0.42 or 42%.

In bacterial transcription, the σ (sigma) factor of RNA polymerase is required for:

A. RNA chain elongation
B. Recognizing promoter sequences
C. DNA replication
D. Termination of transcription

Explanation:
The sigma subunit binds the promoter sequences (e.g., –10 and –35 regions) and directs RNA polymerase to initiate transcription at the correct site.

HIV is a retrovirus that carries which key enzyme for infection of host cells?

A. DNA-dependent DNA polymerase
B. Reverse transcriptase
C. RNA polymerase III
D. Integrase

Explanation:
Retroviruses use reverse transcriptase to copy their RNA genome into DNA, which is then integrated into the host genome.

A one-nucleotide insertion in the coding sequence of a gene will most likely result in which type of mutation?

A. Missense mutation
B. Nonsense mutation
C. Frameshift mutation
D. Silent mutation

Explanation:
Inserting one nucleotide shifts the reading frame of the codons (frameshift), altering all downstream amino acids and usually disrupting function.

On which end of a tRNA molecule is the amino acid attached during translation?

A. 5′ end
B. 3′ end
C. D-loop
D. Anticodon loop

Explanation:
The 3′-terminal CCA sequence of tRNA is where aminoacyl-tRNA synthetases covalently attach the amino acid.